The invention relates in general to a group control for elevators and, in particular, to a group control with protection against overload during immediate allocation of calls of destination.
Many known elevator group control systems include call registering devices arranged on the floors, by means of which calls for desired floors of destination can be entered. The entered calls are stored in floor and car call memories assigned to the elevators of the group where a call characterizing the entry floor is stored in the floor call memory and the calls characterizing the destination floors are stored in the car call register memory. Selectors assigned to each elevator of the group indicate the floor of a possible stop. First and second scanners are assigned to each elevator of the group. The first scanner operates during a cost of operation calculation cycle to store for each floor the costs in a cost memory. The second scanner operates during a cost comparison cycle of the costs for all elevators by means of which the entered call is assigned to the car of the elevator group which exhibits the lowest operating costs.
Such a group control is shown in the European patent application no. EP-A 0 246 395 where the assignments of the cars to the entered calls can be optimized in time. The car call memory of an elevator of this group control consists of a first memory containing assigned destination floor calls and additional memories assigned to the floors, in which the desired floor calls entered at the respective floors, but not yet assigned to a car, are stored. A device, by means of which the entered calls are assigned to the cars of the elevator group, includes a computer in the form of a microprocessor and a comparator device. The computer calculates at each floor, during a scanning cycle of a first scanner, from at least the distance between the floor and the car position indicated by a selector, intermediate stops to be expected within this distance and the load in the car, a sum proportional to the time losses of waiting passengers at the floors and in the car. The car load present at the time of calculation is corrected by factors which correspond to the expected numbers of entering and exiting passengers at future intermediate stops and which have been derived from numbers of passengers entering and exiting in the past. If the first scanners encounter a not yet assigned floor call, then the calls entered at this floor for desired floors of destination, stored in the further memories of the car call memory, are also taken into account. A sum proportional to the new floor calls is therefore determined and a total sum is formed. This total sum, also termed cost of operation, is stored in a cost memory by floor. During a scanning cycle of a second scanner, the operating costs of all elevators are compared with each other by means of the comparator device. An assignment command is stored in an assignment register of the elevator with the lowest operating cost, which designates that floor to which the respective car is optimally assigned in time.
Since, in above described control, the factors on which the calculation of the cost of operation is based are only probable numbers of entering and exiting passengers, which moreover present different values for each elevator of the group, the assignment procedure can lead to inaccurate results. The sum proportional to the time loss of the passengers in the car, also called the internal cost of operation, is used in this control for the determination of a future overload, so that the assignment of a call to the respective car can be prevented promptly. Thus, it is possible that due to the factors used in the calculation of the internal cost of operation, wrong decisions can occur in the assignment of floor calls.